In modern communication systems, for instance for base stations serving cellular mobile telephones, antennas of different states of polarization are utilized. Traditionally vertical polarization is used for both the transmitter and receiver and utilization of dual antennas with a separation distance for obtaining diversity reception. It was also found that a diversity gain was obtained when using two differently polarized antennas together at the same location, typically a horizontal and a vertical polarization, respectively. It has also been found favorable to utilize receive antennas having .+-.45.degree. polarization provided that a good isolation is maintained between them.
In this context there are found several documents involving at least two states of polarization. Several documents are also found which address sequentially rotated elements and the way such actions improve various antenna characteristics.
One document WO 89/08933 and also another document GB, A, 1 572 273 disclose a pair of mutually similar antennas. A number of rectangular portions of microstrip conductors are either directly electrically connected to (WO 89/08933) or electromagnetically connected to (GB, A, 1 572 273) two groups of parallel feed conductors. The small radiators are not placed parallel to the feed but form an angle to a vertical line. By connecting the feeders together with a suitable mutual phase difference dual circular states of polarization are obtained. However, the isolation between the connection ports is not discussed, but the isolation between the elements is apparently to be maintained by the distance between those. Furthermore nothing is mentioned about any particular positioning of the elements.
For antennas polarized for instance .+-.45.degree. it may be an advantage to utilize single polarized elements. Feeding of single polarized radiation elements is easier to arrange with good matching than with dual polarized elements. To be able to handle both the states of polarization in one column the elements then must be closely placed. If according to the state of the art an antenna column is designed having every second element polarized at +45.degree. and every other element polarized at -45.degree. according to FIG. 1 there will be a problem to achieve an isolation of more than 20 dB.
Therefore there is still a desire to find an antenna design, which presents a better isolation than what is achieved in arrays according to the state of the art.